The present invention relates to prototyping of injection molded objects, and more particularly to methods for rapidly making mold tools for use in plastic injection molding prototyping processes.
In a typical injection molding process, plastic is injected at high pressures, extremely quickly, into a thermally conductive metal mold. The molded part is quickly cooled to a temperature at which it can be removed from the mold. The part is then quickly ejected from the mold so that another part can be made, and so that the part does not become stuck on the mold (due to shrink differential). Cooling of large parts continues on a fixture. The goals of production injection modeling are to produce a high quantity of high-quality parts in a short turn-around time. A thirty second cycle time or less for the making of each molded part is typical.
In order to produce a three-dimensional object in a typical injection molding process, it is necessary to prepare a mold tool that has a cavity which is complementary to the desired shape of the three-dimensional object. The mold tool generally consists of two opposing halves, which mate together to define the mold cavity. The mold tool is normally machined out of steel or other metal which is capable of withstanding high temperature and pressure when hot liquid is injected into the mold. In use, the mold tool is inserted into a frame of an injection molding machine, and held in place with high clamping forces to oppose pressure generated inside the mold. The time and skill required to prepare the mold tool are both significant. The machining must be done by skilled craftsmen, and includes the incorporation of a sprue through which the molding material is injected, a vent, cooling lines and ejector pins. Typically, this process involves placing an order with an outside vendor and waiting several weeks or months for delivery, at high cost.
Before undergoing the expense and long lead time associated with conventional metal mold manufacturing, it is desirable to produce a prototype of the part that will have similar characteristics to the production part. The goal is produce a prototype having characteristics sufficiently close to that of the desired final manufactured part so as to permit a close prediction of part performance. Various additive process rapid prototyping (RP) technologies are commonly used to make prototype parts in the design stages of a part. These rapid prototyping technologies include fused deposition modeling (FDM), stereolithography (SLA), selective laser sintering (SLS), laminated object manufacturing (LOM) and jet technology. These additive process techniques produce prototypes useful for evaluating the fit, form and function of a part design, to gain preliminary part approval and to accelerate product development. The strength of a final production part is not, however, replicated in prototypes created by these rapid prototyping techniques. The additive processes create layers, layered stress points and voids in the part resulting in a different internal stress structure than that of the homogeneous injection-molded part. Additionally, many materials used in these processes are weak.
Various methods have been developed for creating mold tools used to make prototype injection molded parts, which may be referred to as “bridge tooling” or “temporary tooling.” A number of these methods utilize rapid prototyping techniques, particularly, stereolithography. For example, U.S. Pat. No. 5,439,622 describes the use of stereolithography to form a mold shell, which is then reinforced with an incompressible material and coated with a thermally conductive material. U.S. Pat. No. 5,989,679 describes a mold tool formed by injecting a strengthening material into cavities within an object formed by stereolithography. U.S. Pat. No. 5,952,018 describes a mold tool, including an ejection valve within the mold tool, formed by stereolithography. U.S. Pat. No. 5,641,448 describes the making of a mold tool by depositing a metal coating onto a plastic mold shell produced by stereolithography.
The use of rapid prototyping to create molds for use in processes other than injection molding are also known. For example, U.S. Pat. No. 6,073,056 describes a mold built by stereolithography or fused deposition modeling used to form a vacuum cast part. U.S. Pat. No. 6,103,156 describes the making of a prototype part by pouring a thermoset into a mold formed by a rapid prototyping technique.
Techniques are also known which use a part formed a rapid prototyping process as a master mold pattern to create a prototype mold tool. For example, U.S. Pat. No. 5,189,781 describes the use of a prototype part as the pattern for making a sprayed metal mold. U.S. Pat. No. 5,707,578 uses a prototype created by stereolithography as a master mold.
A commercial process known as the Swiftool™ process uses a prototype part, which may be made by a rapid prototyping technique, as a pattern for creating bridge tooling. The process takes several days. Another commercial process known as 3D Keltool® makes bridge tooling in a period of several days in a metal-powder sintering process, starting from a master pattern made by stereolithography. Yet another commercial system called AIM™ builds mold tools by stereolithography using UV-sensitive materials.
While the above-described methods do reduce the time and expense of making mold tools, such methods nonetheless require finishing steps which can be tedious and which require additional time and skill to complete. There is a need for a more rapid and low cost method of making a mold tool which can be used to create a small number of prototype injection molded parts.